Continuation of existing curved edges to close mesh body
Select the top edge which you want to extend, hit G twice to activate Edge Slide mode. Now hold the Alt key and you can move the vertices outwards along the projected edges. As soon as your vertices are close enough together press LMB or Return to apply the transformation. Now close the top by hitting F to fill it or  M to merge the vertices. But this extends the edges linear, it doesn't preserve the curvature.